The invention relates to a method for establishing a given print specification defined by a solid color density/dot gain binary value in an autotype printing run which can be influenced by a plurality of printing parameters, on an autotypically operating polychrome printing press with controllable adjusters for influencing one of these printing parameters, namely the feeding of printing inks to adjacent color zones of a material being imprinted in which, for at least one printing ink, solid color densities and dot gain are repeatedly determined by densitometer on patches simultaneously printed within the color zones and the adjusters are regulated accordingly.
The invention further relates to an apparatus on an autotypically operating polychrome printing press with a color density measuring system having at least one densitometer, for the determination of solid color densities and dot gain when the printing press is set up, by the densitometric measurement of simultaneously printed solid patches and screen patches, and with a computer.
Numerous processes and apparatus have been proposed in recent years for the achievement of a uniform printing result (EP No. 0196431, U.S. Pat. No. 3,835,777, GB No. 2,000,082, EP No. 0095649). They serve essentially to permit or to simplify the maintenance of the color balance or at least of preselected color densities in the different color areas, the term "color density" being able to be understood both as a solid color density and as a screen density, i.e., a density value measured on a screen measuring patch.
In addition to the application of such methods and apparatus the need often exists for preparing the printing result also to provide for a given print specification which is defined for each printing color involved, e.g., black, cyan, magenta and yellow, each being defined by a binary number representing the solid color density and the dot gain or screen dot variation. The chief reason for this is to be seen in the fact that identity between proofs on the one hand, which are usually prepared by reproduction houses, and print runs on the other hand, which normally are produced by printing plants, can be achieved only when the above-mentioned binary values are identical in both products. This requirement is, as a rule, not fulfilled and scarcely possible of fulfillment, at least when a relatively great number of companies are involved in producing the printing, as is the case, for example, with the production of periodicals with a great number of color advertisements. In such cases the printing plants and reproduction houses are therefore involved in an agreement on a print specification under which all the participating firms operate in order to assure identity between the proof and the printing run.
Most widespread in this connection for offset printing presses is the "Eurostandard Offset" for the so-called "Commercial Offset Printing" on high-quality, coated papers, and the "Eurostandard Publication" for the printing of periodicals.
The following binary values are used for the "Eurostandard Offset."
______________________________________ Screen Dot Solid Color Enlargement Densities (Dot Gain) ______________________________________ Black DV = 1.50 PV = 17% Cyan DV = 1.30 PV = 15% Magenta DV = 1.40 PV = 15% Yellow DV = 1.30 PV = 15% ______________________________________
Printing according to such a print specification constitutes one of the most difficult problems in working with offset printing presses. The operator of the machine must for this purpose first attempt, during the setting up of the press, to change the one easily influenced printing parameter, namely the feeding of the inks to the individual color areas, by controlling the adjusters (ink valves, area screws, or the like) so that in each individual color area the one or the other of the solid color density/dot gain binary values comes substantially close to the corresponding specification. This is indeed possible in most cases, but it involves some effort, since after each change he makes in an adjuster, the operator must wait for several hundred impressions until the new ink feed has stabilized. When the value corresponding to the specification is finally reached, this does not necessarily mean that the other value of the binary will correspond to its associated standard. Instead the opposite is often found, namely that after the set-up phase described only one of the two values of the binary agrees with the associated specification, while the other value differs unacceptably from its specification and therefore in the end no conformity with the print specification has been obtained.
The operator must therefore now interrupt the printing and attempt, by altering at least one other printing parameter, to adjust the two values of the binary independently of one another in such a degree and direction that finally the desired print standard can be achieved. This is the case with any particular printing order. For even in the case of print orders which are very similar as regards printing plate, paper and printing inks or are even identical and are performed on the same printing press, entirely different dot gains can become established at any selected value of the solid color density. This is dependent upon numerous print parameters, e.g., temperature and atmospheric humidity, the viscosity and/or tack of the printing inks, the different properties of the papers and inks used, in spite of identical quality specifications or the like.
Any change made in this manner in any other printing parameter, i.e., one not relating to the feeding of ink to the color areas is, as a general rule, bound up with the necessity of running again, one or more times, through the above-described process steps for every printing ink and color area involved, until at last the achievement of the print specification is assured. Any abbreviation of this complicated and difficult procedure has not been possible to date, since it cannot be predicted with any assurance what conditions will be when some printing order is performed on the printing press or whether on the basis of this adjustment the stipulated specification on which the test print is based can be obtained at all merely by manipulating the adjusters. Consequently, the procedure described, which involves costly investments in machine time and paper, must either be accepted, or a decision must be made in cooperation with the client in an early phase during the setting up of the printing press, as to whether the print order is to be executed by departing from the print specification agreed upon.
The above-explained, known methods and apparatus can contribute nothing to the solution of the problem described, since they are aimed at the achievement of a uniform print result, and in doing so attempt either to keep given magnitudes constant or to provide recommendations leading to the result best achievable under the given circumstances, but one which does not need to correspond to the given print specification.
The invention is addressed to the problem of further developing the methods and apparatus referred to above to the effect that the operator will be able at a comparatively early moment of time, during the setting up of the printing press, to determine whether the stipulated print specification can be achieved by the mere manipulation of the adjusters, under the circumstances found at the beginning of the execution of a print order.
According to this invention, the method mentioned above is characterized in that at the beginning of the set-up, at least one solid color density/dot gain binary value is tested for conformity with the print specification under consideration of correlations between solid density and dot gain, that the printing run is interrupted if the conformity moves away from a preset tolerance range, that at least one printing parameter other than the one affecting ink feed is changed so as to alter the current "solid color density, dot gain" function, that then the printing run is resumed, and that thereafter, each of these steps is repeated as often as needed until the conformity lies at least in the preselected tolerance range.
The apparatus mentioned above is characterized in accordance with the present invention in that the color density measuring system and the computer are configured and interconnected as parts of a test apparatus intended for testing the printing press for its conformity with a given print specification.
The invention offers two important advantages. On the one hand, it is possible to calculate in advance, by using the correlations known from previous print orders, with the aid of a number of binary values which do not have to be identical to the corresponding specification either in regard to the solid color density or in regard to the dot gain, whether a fairly close approach to the given print specification can be achieved simply by varying the adjusters. Therefore even at a very early time during the set-up phase it can be decided whether the previously calculated departures from the print specification are tolerable or whether the printing process must be interrupted and the achievement of the print specification must be assured by other controllable print parameters. On the other hand, it is not necessary at the beginning of the print to find the specific correlation in the execution of the print order by varying the adjusters. This saves the creation of a great deal of waste paper and the loss of expensive machine time in taking readings which are not at all necessary for judging conformity.
Additional advantageous features of the invention will be found in the following description of a most prefered embodiment of this invention.